1. The Field of the Invention
The present invention relates to a method and casting mold for manufacture of a quartz glass body and to an improved quartz glass body, especially a container or crucible for melting non-metals or non-ferrous metals and especially for manufacture of a silicon ingot or block, from which photovoltaic modules are made.
2. The Related Art
Containers made of sintered quartz glass are predominantly used for making silicon blocks. These containers and/or molded parts and their manufacturing processes are disclosed, for example in DE 102 51 076 or DE 102 44 040. The containers are essentially made with the following process steps:
1. preparing a quartz glass-water mixture, a so-called slip, with quartz of a predetermined gain size (inorganic solid SiO2) and with a predetermined ratio of quartz glass and water;
2. introducing the quartz glass-water mixture into a casting mold, typically a gypsum mold, which has an outer part and an inner part;
3. drying the quartz glass-water mixture in the casting mold, whereby the water is drawn from the quartz glass-water glass mixture and diffuses into the gypsum of the casting mold; and
4. removing the resulting green glass body from the gypsum mold.
Problems always occur according to experience during removal of the green glass body from the gypsum mold, especially when a large-sized container is involved. These problems are handled, for example, in DE 102 51 076. There the use of a container assembled from a number of plates is suggested instead of a finished cast quartz glass container.
Another problem results because it is often desired to provide the container wall that comes in contact with the melted material with a fine-grained textured structure. Because of that the strength of this sort of container suffers. This problem is discussed in DE 102 44 040 and is solved there by building the container up from several layers, which include a fine-grain layer that faces the melted material and a coarse-grained layer next to that layer that faces away from the melted material.
Furthermore a casting mold for making the cast body is disclosed in WO 2006/089754 or EP 1516864 A2. This casting mold has a liquid-impermeable mold wall, in order to prevent water loss through this mold wall.
Finally the green glass body is sintered after removal to form a stable container.
Basically both the hollow casting method and the casting-with-a-core method can be used for casting. Both methods are known and used in the prior art for manufacture of quartz glass crucibles of the above-described type. Normally in the hollow casting method a gypsum mold, which exactly defines the later outer contour of the crucible, is filled with the slip mass. After a predetermined time interval of several minutes to hours, a solid layer that is several mm thick has been deposited on the mold inner surface according to the composition of the slip and the size of the mold. The residual portion of the still flowing slip is removed from the mold and can be used for a subsequent casting. The layer remaining in the mold is subsequently sintered.
In this method the mold must not be completely filled with the slip mass. It is sufficient to partially fill the mold and to completely wet the surfaces of the mold with the slip mass by rotating or swinging the mold for that purpose. Thus a suitable layer is built up on the mold wall.
In an additional variant of this method a mold, which has the inner contour or shape of the container to be made, is immersed in a suitable container with the slip mass until a layer of the desired thickness has been deposited on the outer side of the mold.
The casting-with-a-core method is illustrated in FIGS. 6A to 6C. The slip casting mold has a core 600 and an outer part 602, between which a gap-shaped shaping chamber 604 is formed, in which the slip mass is filled. The walls of the casting mold are conical with an angle 606 for easy removal. The shaping surfaces of the casting mold can be provided in a known manner with a separating agent, in order to prevent the adherence of the dried slip mass with the shaping surfaces. Graphite powder or wax is used as the separating agent. The layer thickness of the separating agent must not prevent or only hinder in an insignificant manner the uptake of water by the mold. It must also be guaranteed that the separating agent does not close the pores of the gypsum mold.
These slip casting molds can be round, square, rectangular, or polygonal. They have a diameter of up to 1200 nm and a height of up to 1400 mm. The wall thickness defined by the gap-shaped shaping chamber 604 amounts to between 6 and 250 nm according to the size of the container to be manufactured. The method of the slip casting mold is typically gypsum or clay in a few cases.
The casting with a central drilled-through core is illustrated in FIG. 6A. The slip mass is filled into the shaping chamber through the central through-going passage 608. At the same time air is removed from the chamber through the gap between the core 600 and the outer part 602. The advantage of filling the mold through a central through-going passage in the core is a uniform distribution of the mass; the highly isotropic mass in the mold leads to uniform shrinkage during sintering. The disadvantage of this casting mold is that the slip must pass over a long flow path within the casting mold and this presupposes a corresponding high castability or pourability. This property must be considered during formulation of the slip.
In the casting mold according to FIG. 6B the slip is fed into the mold through at least two or more openings 610, 612 distributed as symmetrically as possible around the outer surface of the outer part 602. The air in the chamber is removed as before through the gap between the core 600 and the outer part 602. In the simplest variant the introduction of the slip occurs at only one corner (not shown). In that case a very long flow path and a corresponding non-uniform and asymmetrical filling of the mold results. In contrast to that the case shown in FIG. 6B has the problem that a so-called flow front can result at the place where the individual material flows are combined. At this place the bottom of the container that is produced can be inhomogeneous and especially susceptible to crack formation and breakage.
FIG. 6C shows another known embodiment, in which the slip is filled into the mold through a central opening 614 in the bottom of the outer part 602. Again the air removal occurs through the gap between the core 600 and the outer part 602. The advantages and disadvantages are similar to those of the embodiment shown in FIG. 6A, but the flow path is slightly shorter in the case of this embodiment.
There is a further embodiment in which a casting mold of basically the same structure is supplied from over head, which means a core arranged below and an outer part covering over it. In addition to the supply openings air escape openings are provided in the now upper base region of the outer part, since the gap between the core and the outer part is now directed downward and is closed by the slip mass at the start of the slip casting process.
The problems resulting from the prior art methods include air inclusions, which occur when the material flows together (“flow front”), because the air cannot not escape and is included in the material. Furthermore the container formed by the casting method must be put under a reduced load or reduced stress at the flow front location, which also can be caused by contamination of the slip mass by separating agent residues mixed with it. A high castability is required on account of the long flow path. To attain this high castability the slip must have a minimum water content, which is provided by drying the slip mass. Because of that the container produced can have a high porosity and/or a reduced material density. As a result, there is a danger of crack formation because of the drying, which is especially great in the vicinity of material accumulations in the corners or in the bottom region of the container.